Recent work has revealed the existence of a novel class of Zn enzymes, in which the Zn appears to function to activate a coordinated thiolate toward nucleophilic attack. XAS is being used to characterize three such enzymes involved in various aspects of methyl transfer: cobalamin-dependent and cobalamin-independent methionine synthase and cobamide: coenzyme M methyl transferase. EXAFS measurements on the native enzyme, on site directed mutants, and on enzyme samples with a variety of substrates and substrate analogs are used to elucidate the three dimensional geometries of the Zn active sites.